Magnesium alloy sheet and manufacturing method therefor

ABSTRACT

A magnesium alloy sheet containing, relative to 100 wt % of the entire magnesium alloy sheet, 2.7 to 5.0 wt % of Al, 0.75 to 1.0 wt % of Zn, 0.1 to 1.0 wt % of Ca, 1.0 wt % or less of Mn (excluding 0 wt %), and the balance of Mg and other inevitable impurities, wherein a volume fraction of bottom crystal grains, relative to 100 vol % of overall crystal grains of the magnesium alloy sheet, is 30% or less, and the bottom crystal grains are crystal grains in a &lt;0001&gt;//C-axis direction.

TECHNICAL FIELD

An embodiment of the present invention relates to a magnesium alloysheet and a manufacturing method therefor.

BACKGROUND ART

At present, the limitation of carbon dioxide emission and the importanceof renewable energy are becoming a hot issue in the internationalcommunity. Accordingly, lightweight alloys, which are a type ofstructural materials, are recognized as very attractive research fields.

In particular, magnesium is the lightest metal with a density of 1.74g/cm³ and has various advantages such as vibration absorbing ability andelectromagnetic wave shielding ability as compared with other structuralmaterials such as aluminum and steel. Therefore, research of relatedindustry has been actively carried out to utilize magnesium.

An alloy containing magnesium has been currently applied not only in thefield of electronic device but also in the field of vehicle, but it hasfundamental problems in corrosion resistance, flame resistance, andformability, and thus there are limitations in expanding the applicationrange thereof.

In particular, with regard to formability, magnesium has a hexagonalclosed packed (HCP) structure, such that a slip system is not enough atroom temperature, which makes it difficult to perform a processingprocess thereof. That is, a large amount of heat is required in aprocessing process of magnesium, which leads to an increase in the costof the processing process.

Meanwhile, among the magnesium alloys, an AZ-based alloy containsaluminum (Al) and zinc (Zn), and corresponds to a commercializedmagnesium alloy, because it is inexpensive while securing physicalproperties of a somewhat appropriate strength and ductile.

However, the physical properties mentioned above mean only anappropriate level among the magnesium alloys. The strength of theAZ-based alloy is lower than that of aluminum (Al) which is acompetitive material.

Therefore, it is necessary to improve the physical properties such as alow formability and strength of the AZ-based magnesium alloy, but thereis a lack of research on this.

DISCLOSURE Technical Problem

The present invention has been made in an effort to provide a magnesiumalloy sheet and a manufacturing method therefor.

Specifically, the present invention is to improve formability of amagnesium sheet by suppressing center segregation consisting of Al—Casecondary phase particles. Accordingly, the present invention is toprovide a magnesium alloy sheet in which Al—Ca secondary phases aredispersed without being segregated in the center of the magnesium alloysheet.

In addition, the present invention is to improve a strength of amagnesium alloy sheet by controlling a twinned crystal structure throughskin pass rolling while maintaining the formability of the magnesiumalloy sheet. Specifically, a strength of a magnesium alloy sheet may beincreased while maintaining formability of the magnesium alloy sheet byminimizing a development change in texture of (0001) through skin passrolling.

Technical Solution

An exemplary embodiment of the present invention provides a magnesiumalloy sheet containing, relative to 100 wt % of the entire magnesiumalloy sheet, 2.7 to 5.0 wt % of Al, 0.75 to 1.0 wt % of Zn, 0.1 to 1.0wt % of Ca, 1.0 wt % or less of Mn (excluding 0 wt %), and the balanceof Mg and other inevitable impurities, wherein a volume fraction ofbottom crystal grains, relative to 100 vol % of overall crystal grainsof the magnesium alloy sheet, is 30% or less, and the bottom crystalgrains are crystal grains in a <0001>//C-axis direction.

The magnesium alloy sheet may include Al—Ca secondary phase particles,and a difference in area fraction of the Al—Ca secondary phase particlesbetween a quarter portion (¼) of a surface of the magnesium alloy sheetand a center portion (½) of the surface of the magnesium alloy sheet maybe 10% or less.

Specifically, a ratio of a length of center segregation to a totallength of the magnesium alloy sheet in a rolling direction may be lessthan 5%.

A ratio of a thickness of the center segregation to a total thickness ofthe magnesium alloy sheet in a thickness direction may be less than2.5%. Therefore, in the magnesium alloy sheet, the Al—Ca secondary phaseparticles may be uniformly distributed without being segregated in thecenter portion of the magnesium alloy sheet.

The Al—Ca secondary phase particle may contain, relative to 100 wt % ofthe entire Al—Ca secondary phase particle, 20.0 to 25.0 wt % of Al, 5.0to 10.0 wt % of Ca, 0.1 to 0.5 wt % of Mn, 0.5 to 1.0 wt % of Zn, andthe balance of Mg and other inevitable impurities.

An average particle size of the Al—Ca secondary phase particles may be0.01 to 4 μm.

2 to 15 Al—Ca secondary phase particles may be included per area of 100μm² of the magnesium alloy sheet.

A limiting dome height (LDH) of the magnesium alloy sheet may be 7 mm ormore.

A maximum texture intensity of a (0001) surface of the magnesium alloysheet may be 1 to 4.

A yield strength of the magnesium alloy sheet may be 150 to 190 MPa.

Another exemplary embodiment of the present invention provides amagnesium alloy sheet containing: relative to 100 wt % of the entiremagnesium alloy sheet, 2.7 to 5.0 wt % of Al, 0.75 to 1.0 wt % of Zn,0.1 to 1.0 wt % of Ca, 1.0 wt % or less of Mn (excluding 0 wt %), andthe balance of Mg and other inevitable impurities, wherein a volumefraction of a twinned crystal structure, relative to 100 vol % of theentire area of the magnesium alloy sheet, is 35% or less.

Specifically, the volume fraction of the twinned crystal structure,relative to 100 vol % of the entire area of the magnesium alloy sheet,may be 5 to 35%.

The magnesium alloy sheet in which a volume fraction of bottom crystalgrains, relative to 100 vol % of overall crystal grains of the magnesiumalloy sheet, is 30% or less, and the bottom crystal grains are crystalgrains in a <0001>//C-axis direction may be provided.

A limiting dome height of the magnesium alloy sheet may be 7 mm or more.

A maximum texture intensity of a (0001) surface of the magnesium alloysheet may be 1 to 4.

A yield strength of the magnesium alloy sheet may be 200 to 300 MPa.

Advantageous Effect

In the magnesium alloy sheet according to an embodiment of the presentinvention, the center segregations consisting of Al—Ca secondary phaseparticles are dispersed, such that the formability the magnesium sheetmay be improved. Accordingly, according to an embodiment of the presentinvention, it is possible to provide the magnesium alloy sheet in whichthe Al—Ca secondary phases are dispersed without being segregated in thecenter of the magnesium alloy sheet. Specifically, it is possible toprovide the magnesium alloy sheet in which a difference in area fractionof the Al—Ca secondary phase particles between a quarter portion (¼) ofa surface of the magnesium alloy sheet and a center portion (½) of thesurface of the magnesium alloy sheet is 10% or less. According to anembodiment of the present invention, it is possible to obtain, throughskin pass rolling, the magnesium alloy sheet in which an area fractionof a twinned crystal structure, relative to 100% of the entire area ofthe magnesium alloy sheet, is 35% or less. Specifically, the strength ofthe magnesium alloy sheet may be improved by controlling the twinnedcrystal structure while maintaining the formability of the magnesiumalloy sheet by minimizing the development of a texture of (0001) througha skin pass process.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart schematically illustrating a manufacturing methodfor a magnesium alloy sheet according to an embodiment of the presentinvention.

FIG. 2 is a photograph obtained by observing a magnesium alloy sheetproduced in Example 1a with optical microscopy.

FIG. 3 is a photograph obtained by observing a magnesium alloy sheetproduced in Comparative Example 1a with optical microscopy.

FIG. 4 is a photograph obtained by observing the magnesium alloy sheetproduced in Example 1a with secondary electron microscopy.

FIG. 5 shows a result of measuring a limiting dome height of themagnesium alloy sheet produced in Example 1a.

FIG. 6 shows a maximum texture intensity of a (0001) surface of Example1a.

FIG. 7 shows a maximum texture intensity of a (0001) surface ofComparative Example 1a.

FIG. 8 shows a result of electron backscatter diffraction (EBSD)analysis of the magnesium alloy sheet produced in Example 1a.

FIG. 9 is a graph illustrating fractions of crystal orientations ofExample 1a.

FIG. 10 is a result of EBSD analysis of a magnesium alloy sheetaccording to a reduction ratio of skin pass.

FIG. 11 shows a maximum texture intensity of each of (0001) surfaces ofExample 2 and Comparative Example 2, depending on a skin pass condition.

MODE FOR INVENTION

The advantages and features of the present invention, and methods ofaccomplishing these will become obvious with reference to theembodiments to be described below in detail along with the accompanyingdrawings. However, the present invention is not limited to embodimentsto be disclosed below, but various forms different from each other maybe implemented. The embodiments are merely provided to make the presentinvention complete and to completely notify those skilled in the art towhich the present invention pertains, of the scope of the presentinvention, and the present invention is only defined by the scope of theclaims. The same reference numerals throughout the specification denotethe same elements.

Accordingly, in some embodiments, well-known techniques are notdescribed in detail in order to avoid obscuring the present invention.Unless otherwise defined, all terms (including technical and scientificterms) used in the specification have the same meaning as commonlyunderstood by those skilled in the art to which the present inventionpertains. Throughout the present specification, unless explicitlydescribed to the contrary, “comprising” any components will beunderstood to imply the inclusion of other elements rather than theexclusion of any other elements. In addition, unless explicitlydescribed to the contrary, a singular form includes a plural form.

A magnesium alloy sheet according to an embodiment of the presentinvention may contain, relative to 100 wt % of the entire magnesiumalloy sheet, 2.7 to 5.0 wt % of Al, 0.75 to 1.0 wt % of Zn, 0.1 to 1.0wt % of Ca, 1.0 wt % or less of Mn (excluding 0 wt %), and the balanceof Mg and other inevitable impurities.

Hereinafter, the reason for limiting the components and compositionswill be described.

First, aluminum (Al) improves the mechanical properties of the magnesiumalloy sheet and castability of a molten metal. When Al is added in anamount of more than 5.0 wt %, the castability may rapidly deteriorate.On the other hand, when Al is added in an amount of less than 2.7 wt %,the mechanical properties of the magnesium alloy sheet may deteriorate.Therefore, a content of Al may be adjusted within the above-mentionedrange.

Zinc (Zn) improves the mechanical properties of the magnesium alloysheet. When Zn is added in an amount of more than 1.0 wt %, a largenumber of surface defects or center segregations is generated, and thecastability may thus rapidly deteriorate. On the other hand, when Zn isadded in an amount of less than 0.75 wt %, the mechanical properties ofthe magnesium alloy sheet may deteriorate. Therefore, a content of Znmay be adjusted within the above-mentioned range.

Calcium (Ca) imparts flame resistance to the magnesium alloy sheet. WhenCa is added in an amount of more than 1.0 wt %, the castability mayrapidly deteriorate due to reduction of fluidity of the molten metal,and the formability of the magnesium alloy sheet may deteriorate due toan increase of the center segregation consisting of an Al—Ca-basedintermetallic compound. When Ca is added in an amount of less than 0.1wt %, the flame resistance may not be sufficiently imparted. Therefore,a content of Ca may be adjusted within the above-mentioned range. Morespecifically, Ca may be contained in an amount of 0.5 to 0.8 wt %.

Manganese (Mn) improves the mechanical properties of the magnesium alloysheet. When Mn is added in an amount of more than 1.0 wt %, a heatdissipation property may deteriorate and uniform distribution controlmay be difficult. Therefore, a content of Mn may be adjusted within theabove-mentioned range.

The volume fraction of bottom crystal grains, relative to 100 vol % ofoverall crystal grains of the magnesium alloy sheet may be 30% or less.

In an embodiment of the present invention, a bottom crystal grain refersto a crystal grain with a bottom orientation. Specifically, magnesiumhas a hexagonal closed pack (HCP) crystal structure, here, a crystalgrain when a C-axis of the crystal structure is parallel to a thicknessdirection of the sheet refers to a crystal grain with a bottom crystalorientation (that is, bottom crystal grain). Accordingly, in the presentspecification, the bottom crystal grain may be expressed as a“<0001>//C-axis”.

More specifically, in a case where a fraction of the bottom crystalgrains is in the above-mentioned range, a magnesium alloy sheet havingan excellent formability may be obtained.

Specifically, the volume fraction of bottom crystal grains in the<0001>//C-axis direction, relative to 100 vol % of overall crystalgrains of the magnesium alloy sheet may be 30% or less. Morespecifically, the volume fraction of bottom crystal grains in the<0001>//C-axis direction, relative to 100 vol % of overall crystalgrains of the magnesium alloy sheet may be 25% or less. Still morespecifically, the volume fraction of bottom crystal grains in the<0001>//C-axis direction, relative to 100 vol % of overall crystalgrains of the magnesium alloy sheet may be 20% or less. A lower limit ofthe volume fraction of bottom crystal grains in the <0001>//C-axis maybe more than 0%. This means that in a case where the volume fraction ofcrystal grains in the <0001>//C-axis direction is in any range (morethan 0%), the magnesium alloy sheet may be included in the presentinvention.

In the magnesium alloy sheet, the fraction of crystal grains in the<0001>//C-axis direction may be decreased due to an increase of theorientation distribution of crystal grains.

In a case where the fraction of crystal grains in the <0001>//C-axisdirection satisfies the above-mentioned range, the texture intensity ofthe magnesium alloy sheet is decreased, such that a magnesium alloysheet having an excellent formability may be obtained.

The magnesium alloy sheet according to an embodiment of the presentinvention may include Al—Ca secondary phase particles.

Specifically, the magnesium alloy sheet according to an embodiment ofthe present invention may include the Al—Ca secondary phase particles,but may hardly include the center segregation. More specifically, themagnesium alloy sheet according to an embodiment of the presentinvention may have a form in which the Al—Ca secondary phase particlesare uniformly dispersed. The center segregation refers to that Al—Casecondary phase particles are segregated in the center portion of themagnesium alloy sheet in the thickness direction (ND), and as describedabove, as the center segregation increases, the formability of themagnesium alloy sheet may deteriorate.

Accordingly, in the magnesium alloy sheet according to an embodiment ofthe present invention, a difference in area fraction of the Al—Casecondary phase particles between a quarter portion (¼) of a surface ofthe magnesium alloy sheet and the center portion (½) of the surface ofthe magnesium alloy sheet may be 10% or less. Therefore, the Al—Casecondary phase particles are entirely and uniformly dispersed withoutsegregation in the center portion of the magnesium alloy sheet, and theformability of the magnesium alloy sheet may thus be improved. Here, thearea fraction refers to a fraction with respect to an area of the Al—Casecondary phase particles per same area of the quarter portion and thecenter portion.

More specifically, a ratio of a length of the center segregation to thetotal length of the magnesium alloy sheet in the rolling direction (RD)may be less than 5%. In addition, a ratio of a thickness of the centersegregation to the total thickness of the magnesium alloy sheet in thethickness direction (ND) may be less than 2.5%.

The above description means that the center segregation is hardlygenerated, and the above range is a range in which both the length andthickness of the center segregation are decreased as compared to centersegregation which is generally generated when Al and Ca are added.Therefore, the formability of the magnesium alloy sheet according to anembodiment of the present invention may be improved.

The total length of the magnesium sheet may be based on a magnesiumsheet with a constant length unit. Specifically, the length unit may be1,000 to 3,000 μm.

Specifically, the Al—Ca secondary phase particle may contain, relativeto 100 wt % of the entire Al—Ca secondary phase particle, 20.0 to 25.0wt % of Al, 5.0 to 10.0 wt % of Ca, 0.1 to 0.5 wt % of Mn, 0.5 to 1.0 wt% of Zn, and the balance of Mg and other inevitable impurities.

In general, in a case where Al and Ca are added to magnesium to from analloy, center segregation consisting of Al—Ca secondary phase particlesis generated, which causes the significant deterioration of theformability of the magnesium alloy sheet. On the other hand, themagnesium alloy sheet according to an embodiment of the presentinvention may improve the formability of the magnesium sheet bysuppressing the generation of the center segregation consisting of Al—Casecondary phase particles. Specifically, the magnesium alloy sheet inwhich Al—Ca secondary phase particles are dispersed may be provided.

An average particle size of the Al—Ca secondary phase particles may be0.01 to 4 μm. As the average particle size of the Al—Ca secondary phaseparticles is large, as described above, the formability of the magnesiumalloy sheet may deteriorate due to the generation of the centersegregation. Within the above-mentioned range of the particle size, theimproved formability may be exhibited.

2 to 15 Al—Ca secondary phase particles may be included per area of 100μm² of the magnesium alloy sheet. The number of Al—Ca secondary phaseparticles is in the above-mentioned range, the formability of themagnesium alloy sheet may be improved.

In an embodiment of the present invention, in order to control the Al—Casecondary phase particles, composition ranges of Al, Zn, Mn, and Ca,temperature and time conditions during homogenization heat treatment,temperature and rolling ratio during warm-rolling, and the like may beprecisely controlled.

The magnesium alloy sheet according to an embodiment of the presentinvention includes crystal grains, and an average particle size of thecrystal grains may be 5 to 30 μm. Within the above particle size rangeof the crystal grains, the formability of the magnesium alloy sheet maybe improved.

In addition, a limiting dome height of the magnesium alloy sheetaccording to an embodiment of the present invention may be 7 mm or more.More preferably, the limiting dome height of the magnesium alloy sheetmay be 7 to 10 mm.

In general, a limiting dome height is utilized as an index forevaluating formability (in particular, pressability) of a material, andas the limiting dome height is increased, the formability of thematerial is improved.

A limiting dome height within the above limited range is a significantlyhigher limiting dome height than that of a magnesium alloy sheetgenerally known, which caused by an increase in orientation distributionof the crystal grain in the magnesium alloy sheet.

Therefore, the maximum texture intensity of a (0001) surface of themagnesium alloy sheet may be 1 to 4. In a case where the limiting domeheight is out of the above-mentioned range, the formability of themagnesium alloy sheet may deteriorate.

In addition, a yield strength of the magnesium alloy sheet according toan embodiment of the present invention may be in a range of 150 to 190MPa.

In the magnesium alloy sheet according to an embodiment of the presentinvention, through skin pass in a production step described below, anarea fraction of a twinned crystal structure may be 35% or less relativeto 100% of the entire area of the magnesium alloy sheet. Morespecifically, the area fraction of the twinned crystal structure may be5 to 35%. Still more specifically, the area fraction of the twinnedcrystal structure may be 5 to 33%. By controlling the fraction of thetwinned crystal structure to the above range, the yield strength of themagnesium alloy sheet according to an embodiment of the presentinvention may be 200 to 300 MPa. This range is considered as anexcellent range in the magnesium sheet according to an embodiment of thepresent invention.

In addition, a thickness of the magnesium alloy sheet according to anembodiment of the present invention may be 0.4 to 3 mm. The magnesiumsheet according to an embodiment of the present invention may beselected depending on properties required in the above thickness range.However, the present invention is not limited to this thickness range.

FIG. 1 is a flowchart schematically illustrating a manufacturing methodfor a magnesium alloy sheet according to an embodiment of the presentinvention. The flowchart of the manufacturing method for a magnesiumalloy sheet of FIG. 1 is merely to illustrate the present invention, andthe present invention is not limited thereto. Therefore, themanufacturing method for a magnesium alloy sheet may be variouslymodified.

A manufacturing method for a magnesium alloy sheet according to anembodiment of the present invention includes: a step (S10) of preparinga cast material by casting a molten metal, the molten metal containing,relative to 100 wt % of the entire molten metal, 2.7 to 5.0 wt % of Al,0.75 to 1.0 wt % of Zn, 0.1 to 1.0 wt % of Ca, 1.0 wt % or less of Mn(excluding 0 wt %), and the balance of Mg and other inevitableimpurities; a step (S20) of subjecting the cast material tohomogenization heat treatment; and a step (S30) of subjecting the castmaterial subjected to the homogenization heat treatment to warm-rolling.

In addition, the manufacturing method for a magnesium alloy sheet mayfurther include other steps, as necessary.

First, the step (S10) of preparing a cast material by casting a moltenmetal may be performed, the molten metal containing, relative to 100 wt% of the entire molten metal, 2.7 to 5.0 wt % of Al, 0.75 to 1.0 wt % ofZn, 0.1 to 1.0 wt % of Ca, 1.0 wt % or less of Mn (excluding 0 wt %),and the balance of Mg and other inevitable impurities.

The reason for limiting the numeral values of the respective componentsis the same as that mentioned above, and the repeated descriptionthereof will thus be omitted.

At this time, as the method (S10) of preparing the cast material, a diecasting method, a strip casting method, a billet casting method, acentrifugal casting method, a tilting casting method, a sand castingmethod, a direct chill casting method, or combination thereof may beused.

More specifically, a strip casting method may be used. However, thepresent invention is not limited thereto.

More specifically, in the step (S10) of preparing the cast material, arolling force may be 0.2 ton/mm² or more. Still more specifically, therolling force may be 1 ton/mm² or more. Still more specifically, therolling force may be 1.5 ton/mm². The cast material is coagulated and arolling force is simultaneously applied thereto, and at this time, theformability of the magnesium alloy sheet may be improved by adjustingthe rolling force to the above range.

Next, the step (S20) of subjecting the cast material to homogenizationheat treatment may be carried out.

At this time, the heat treatment may be performed at a temperature of350° C. to 500° C. for 1 to 28 hours. More specifically, thehomogenization heat treatment may be performed for 18 to 28 hours.

In a temperature range of lower than 350° C., the homogenization heattreatment is not properly performed, and beta phases such as Mg₁₇Al₂ maynot be solid-dissolved in the matrix.

In a temperature range of higher than 500° C., the beta phases condensedin the cast material may melt, resulting in an occurrence of a fire orformation of holes in the magnesium sheet. Therefore, the homogenizationheat treatment may be performed within the above-mentioned temperaturerange.

Next, the step (S30) of subjecting the cast material subjected to thehomogenization heat treatment to warm-rolling may be carried out.

At this time, a temperature condition of the warm-rolling may be 150° C.to 350° C. In a temperature range of lower than 150° C., a large amountof edge cracks may be generated. In a temperature range of higher thanof 500° C., the magnesium alloy sheet may not be appropriate for massproduction. Therefore, the warm-rolling may be performed in theabove-mentioned temperature range.

The step of subjecting the cast material subjected to the homogenizationheat treatment to warm-rolling may be carried out a plurality of times,and the warm-rolling may be performed at a reduction ratio of 10 to 30%per time. The reduction ratio of the warm-rolling refers to a “value(%)” relative to 100% of the thickness (length (%) of the cast material.By performing warm-rolling a plurality of times, finally, the rollingmay be performed until the cast material has a thin thickness of about0.4 mm.

At least one time of a step of performing intermediate annealing in themiddle of a plurality of times of warm-rolling may be further included.By further including the step of performing intermediate annealing inthe middle of a plurality of times of warm-rolling, the formability ofthe magnesium alloy sheet may be further improved. Specifically, thestep of performing intermediate annealing may be carried out at 300 to500° C. for 1 to 10 hours. More specifically, the intermediate annealingstep may be carried out at 450 to 500° C. Within the above-mentionedrange, the formability of the magnesium alloy sheet may be furtherimproved.

After the warm-rolling step, the method may further include a step ofperforming subsequent heat treatment. By including the step ofperforming subsequent heat treatment, the formability of the magnesiumalloy sheet may be further improved. The step of performing subsequentheat treatment may be carried out at 300 to 500° C. for 1 to 15 hours.Specifically, the step of performing subsequent heat treatment may becarried out for 1 to 10 hours. Within the above-mentioned range, theformability of the magnesium alloy sheet may be further improved.

A manufacturing method for a magnesium alloy sheet according to anotherembodiment of the present invention may include: a step of preparing acast material by casting a molten metal, the molten metal containing,relative to 100 wt % of the entire molten metal, 2.7 to 5.0 wt % of Al,0.75 to 1.0 wt % of Zn, 0.1 to 1.0 wt % of Ca, 1.0 wt % or less of Mn(excluding 0 wt %), and the balance of Mg and other inevitableimpurities; a step of subjecting the cast material to homogenizationheat treatment; a step of preparing a rolled material by subjecting thecast material subjected to the homogenization heat treatment towarm-rolling; a step of subjecting the rolled material to subsequentheat treatment; and a step of producing a magnesium alloy sheet bysubjecting the rolled material subjected to the subsequent heattreatment to skin pass.

First, the step of preparing a cast material by casting a molten metalmay be performed, the molten metal containing, relative to 100 wt % ofthe entire molten metal, 2.7 to 5.0 wt % of Al, 0.75 to 1.0 wt % of Zn,0.1 to 1.0 wt % of Ca, 1.0 wt % or less of Mn (excluding 0 wt %), andthe balance of Mg and other inevitable impurities.

In the above step, the molten metal may a commercially available AZ31alloy, AL5083 alloy, or a combination thereof. However, the presentinvention is not limited thereto.

More specifically, the molten metal may be prepared in a temperaturerange of 650 to 750° C. Thereafter, a cast material may be produced bycasting the molten metal. At this time, a thickness of the cast materialmay be 3 to 7 mm.

At this time, as the method of preparing the cast material, a diecasting method, a strip casting method, a billet casting method, acentrifugal casting method, a tilting casting method, a sand castingmethod, a direct chill casting method, or combination thereof may beused. More specifically, a strip casting method may be used. However,the present invention is not limited thereto.

More specifically, in the step of preparing the cast material, a rollingforce may be 0.2 ton/mm² or more. Still more specifically, the rollingforce may be 1 ton/mm² or more. Still more specifically, the rollingforce may be 1.5 ton/mm².

Next, the step of subjecting the cast material to homogenization heattreatment may be carried out.

More specifically, the step of subjecting the cast material tohomogenization heat treatment may include: a primary heat treatment stepin a temperature range of 300° C. to 400° C. and a secondary heattreatment step in a temperature range of 400° C. to 500° C. Thetemperature ranges of the primary heat treatment step and the secondaryheat treatment step may be different from each other.

Still more specifically, the primary heat treatment step in atemperature range of 300° C. to 400° C. may be carried out for 5 hoursto 20 hours. In addition, the secondary heat treatment step in atemperature range of 400° C. to 500° C. may be carried out for 5 hoursto 20 hours.

By carrying out the primary heat treatment step in the above temperaturerange, a Mg—Al—Zn ternary system Pi-phase generated in the casting stepmay be removed. In a case where the ternary system Pi-phase is present,the subsequent process may be adversely affected. In addition, bycarrying out the secondary heat treatment step in the above temperaturerange, a stress in a slab may be released. Further, the formation ofrecrystallization of the cast structure may be more actively induced.

Next, the step of preparing a rolled material by subjecting the castmaterial subjected to the homogenization heat treatment to warm-rollingmay be carried out.

The cast material subjected to the heat treatment may be rolled to athickness range of 0.4 to 3 mm through 1 to 15 times of rolling. Inaddition, the rolling may be performed at 150 to 350° C.

More specifically, in a case where the rolling temperature is lower than150° C., a crack on the surface when rolling may be induced, and in acase where the rolling temperature is higher than 350° C., it may not besuitable for actual production facilities. Therefore, the rolling may beperformed at 150° C. to 350° C.

Next, a step of subjecting the rolled material to intermediate annealingmay be carried out. In the rolling step, when the cast material isrolled a plurality of times, heat treatment may be performed in atemperature range of 300° C. to 550° C. for 1 hour to 15 hours in aninterval between the pass and the pass.

For example, the intermediate annealing is performed one time afterperforming the rolling two times, and the rolled material may thus berolled to the final target thickness. As another example, theintermediate annealing is performed one time after performing therolling three times, and the rolled material may thus be rolled to thefinal target thickness. More specifically, in a case where the rolledcast material is annealed in the above temperature range, the stressgenerated by the rolling may be released. Therefore, the rolling may beperformed several times to obtain a desired thickness of the castmaterial.

Next, the step of subjecting the rolled material to subsequent heattreatment may be carried out.

The step of subjecting the cast material to subsequent heat treatmentmay be carried out at 300 to 500° C. for 1 to 15 hours. Specifically,the step of subjecting the cast material to subsequent heat treatmentmay be carried out for 1 to 10 hours. Within the above-mentioned range,the formability of the magnesium alloy sheet may be further improved.

Finally, the step of producing a magnesium alloy sheet by subjecting therolled material subjected to the subsequent heat treatment to skin passmay be carried out.

More specifically, the skin pass is also referred to as skin passrolling or temper rolling, which means that a deformation patterngenerated in a cold rolled sheet after heat treatment is removed, andcold rolling is performed with a light pressure to improve the strength.

Therefore, in an embodiment of the present invention, the skin pass maybe performed one time in a temperature range of 250° C. to 350° C.

The magnesium alloy sheet produced by performing the skin pass may berolled at a reduction ratio of 2 to 15% with respect to the thickness ofthe rolled material. More specifically, the reduction ratio may berelated to the skin pass temperature.

As a specific example, when the skin pass temperature is 250° C., thereduction ratio of the skin pass may be 5 to 15%. At this time, a yieldstrength may be in a range of 200 to 260 MPa. Further, at this time, alimiting dome height may be in a range of 7.3 to 8.1.

As a specific example, when the skin pass temperature is 300° C., thereduction ratio of the skin pass may be 5 to 15%. More preferably, thereduction ratio of the skin pass may be 7 to 12%. At this time, a yieldstrength may be in a range of 200 to 250 MPa. Further, at this time, alimiting dome height may be in a range of 7.3 to 8.1.

In the present invention, a limit dome height (LDH) is an index forevaluating formability of the sheet, in particular, pressability, andthe formability may be measured by measuring a deformed height of aspecimen obtained by applying a deformation to the specimen. A highvalue of the limiting dome height means that the formability of thesheet is excellent.

More specifically, the skin pass is performed under the conditions ofthe above temperature and pressure, the development of the texture of(0001) is suppressed, the formability may be secured. That is, in a casewhere the skin pass is performed under the above conditions, a change ofthe texture intensity may be minimized and the strength may thus beincreased.

Hereinafter, the present invention will be described in detail withreference to examples. However, the following examples are only toillustrate the present invention, and the contents of the presentinvention are not limited by the following examples.

Example 1

A molten metal containing, relative to 100 wt % of the entire moltenmetal, Al and Ca in amounts as shown in Table 1, 0.8 wt % of Zn, 0.5 wt% of Mn, and the balance of Mg and inevitable impurities was prepared.

The molten metal was passed between two cooling rolls to prepare amagnesium cast material. At this time, a rolling force of the coolingroll is as shown in Table 1.

Next, the magnesium cast material was subjected to homogenization heattreatment at 400° C. while varying time as shown in Table 1.

The magnesium cast material subjected to the homogenization heattreatment was subjected to warm-rolling at a temperature of 250° C. at areduction ratio of 15%. Next, the magnesium cast material subjected tothe warm-rolling was subjected to intermediate annealing at atemperature as shown in Table 1, and then subjected to warm-rollingagain at a temperature of 250° C. at a reduction ratio of 15%, therebyproducing a magnesium alloy sheet.

Comparative Example 1

A molten metal containing, relative to 100 wt % of the entire moltenmetal, Al and Ca in amounts as shown in Table 1, 0.8 wt % of Zn, and thebalance of Mg and inevitable impurities was prepared.

A magnesium alloy sheet was produced in the same manner as that ofExample 1, except for the conditions as shown in Table 1.

TABLE 1 Casting roll Intermediate Al Ca Rolling Homogenization Rollingannealing content content force Annealing time temperature temperature(wt %) (wt %) (ton/mm²) (hr) (C.) (C.) Example 1a 3 0.6 1.2 24 250 450Example 1b 4 0.6 1.2 24 250 450 Example 1c 5 0.6 1 24 250 450 Example 1d3 0.6 1.2 24 250 300 Example 1e 3 0.6 1.2 24 250 400 Example 1f 3 0.61.2 24 250 500 Example 1g 3 0.7 0.2 24 250 500 Example 1h 3 0.7 1.2 24250 450 Example 1i 3 0.6 1 1 250 400 Comparative 3 0.6 0.8 24 250 —Example 1a Comparative 3 0.7 1.2 24 400 250 Example 1b Comparative 3 0.71 48 250 400 Example 1c Comparative 3 0.7 0.8 24 100 400 Example 1d

In order to compare and evaluate physical properties of the magnesiumalloy sheets produced in examples and comparative examples, thefollowing experimental examples were performed.

Experimental Example 1: Observation of Microstructure of Magnesium AlloySheet

Microstructures of the magnesium alloy sheets produced in examples andcomparative examples were observed with a scanning electron microscope(SEM).

The observed results are illustrated in FIGS. 2 to 4 of the presentinvention.

FIG. 2 is a photograph obtained by observing a magnesium alloy sheetproduced in Example 1a with a scanning electron microscope (SEM). FIG. 3is a photograph obtained by observing a magnesium alloy sheet producedin Comparative Example 1a with a scanning electron microscope (SEM).

Specifically, in each of FIGS. 2 and 3, a horizontal direction is arolling direction (RD) of the magnesium alloy sheet and a verticaldirection is a thickness direction (ND) of the magnesium alloy sheet.

As illustrated in FIG. 2, it can be appreciated that center segregationof the magnesium alloy sheet was hardly generated in Example 1a.Specifically, it can be appreciated that a ratio of a length of thecenter segregation to the total length of about 2000 μm in the rollingdirection in Example 1a was less than 5%.

On the other hand, as illustrated in FIG. 3, it can be appreciated that,a large amount of center segregation of the magnesium alloy sheet wasgenerated in Comparative Example 1a. Specifically, it can be appreciatedthat a ratio of a length of the center segregation to the total lengthof about 2000 μm in the rolling direction in Comparative Example a was5% or more. Further, it was confirmed that, a thickness of the centersegregation to the total thickness of about 1200 μm in the thicknessdirection in Comparative Example 1a was about 30 μm. From this fact, itcan be appreciated that a ratio of the thickness of the centersegregation to the total thickness of the magnesium alloy sheet in thethickness direction was 2.5%. Therefore, it could be confirmed that alarge amount of center segregation was generated in Comparative Example1a.

As described above, since the center segregation causes deterioration offormability of the magnesium alloy sheet, as the center segregation isnot generated, a magnesium alloy sheet having an excellent formabilitymay be obtained.

FIG. 4 is a photograph obtained by observing the magnesium alloy sheetproduced in Example 1a with secondary electron microscopy.

The white dots in FIG. 4 are Al—Ca secondary phase particles. Morespecifically, as a result of analyzing compositions of the white dots inFIG. 4, it was analyzed that the white dots contain 24.61 wt % of Al,8.75 wt % of Ca, 0.36 wt % of Mn, 0.66 wt % of Zn, and the balance of Mgand other inevitable impurities.

From this fact, it was confirmed that the magnesium alloy sheet producedin Example 1a includes the Al—Ca secondary phase particles.Specifically, it can be appreciated that 50 Al—Ca secondary phaseparticles were distributed per area of 1600 μm² of the magnesium alloysheet in FIG. 4.

As illustrated in FIG. 4, it can be appreciated that the centersegregation of the Al—Ca secondary phase particles was not generated inExample 1a, and the Al—Ca secondary phase particles were dispersed. Fromthis fact, as shown in Table 2, it can be appreciated that a limitingdome height of the magnesium alloy sheet produced in Example 1a of thepresent invention is 9.4 mm, whereas a limiting dome height of themagnesium alloy sheet produced in Comparative Example 1a is 2.5 mm,which shows that the formability of the magnesium alloy sheet producedin Comparative Example 1a is inferior to that of the magnesium alloysheet produced in Example 1a.

Experimental Example 2: Measurement of Limiting Dome Height of MagnesiumAlloy Sheet

In the present invention, a limit dome height (LDH) is an index forevaluating formability of the sheet, in particular, pressability, andthe formability may be measured by measuring a deformed height of aspecimen obtained by applying a deformation to the specimen.

The limiting dome height was measured by inserting each of the magnesiumalloy sheets of examples and comparative examples between an upper dieand a lower die, and fixing an outer periphery of each specimen with aforce of 5 kN. Here, a known press oil was used as a lubricant. Then, aspherical punch having a diameter of 20 mm was used to deform thespecimen at a rate of 5 to 10 mm/min, the punch was inserted until eachspecimen was fractured, and then a deformed height of each specimen atthe time of fracturing was measured. That is, the deformed height of thespecimen was measured.

The results are illustrated in FIG. 5 of the present invention.

FIG. 5 shows a result of measuring a limiting dome height of themagnesium alloy sheet produced in Example 1a.

As illustrated in FIG. 5, it can be appreciated that the magnesium alloysheet produced in Example 1a has an excellent formability.

The results can also be confirmed in Tables 2 and 3.

Experimental Example 3: Analysis of Crystal Orientation of Crystal Grain

The crystal orientation of crystal grain of each of the magnesium alloysheets produced in examples and comparative examples were confirmed withan XRD analyzer, and the results are illustrated in FIGS. 6 to 11.Specifically, a texture of the crystal grains obtained by using an XRDpole figure method is illustrated.

More specifically, the pole figure is represented by stereographicprojection of an orientation of an arbitrarily fixed crystal coordinatesystem onto a coordinate system of the specimen. Still morespecifically, poles of the crystal grains with various orientations withrespect to a {0001} surface are represented on a standard coordinatesystem, and a density contour of the poles is drawn according to a poledensity distribution, thereby representing the pole figure. At thistime, the poles are fixed in a specific lattice direction by Bragg'sangle, and a plurality of poles may be represented for a single crystal.

Accordingly, it can be construed that as a density distribution value ofthe contour represented by the pole figure method is small, crystalgrains with various orientations are distributed, and as the densitydistribution value is large, a large amount of crystal grains in a<0001>/C-axis direction is distributed.

The results may be compared through FIGS. 6 and 7 of the presentinvention.

FIG. 6 shows a maximum texture intensity of a (0001) surface of Example1a. FIG. 7 shows a maximum texture intensity of a (0001) surface ofComparative Example 1a.

Specifically, the maximum texture intensity of each of the (0001)surfaces of FIGS. 6 and 7 is the result obtained by analyzing thecrystal orientation of the magnesium alloy sheet with the XRD analyzeras described above.

As illustrated in FIG. 6, it could be confirmed that the maximum densitydistribution value (texture intensity) of the (0001) surface in exampleswas 2.73, which is low, whereas the maximum density distribution valuein comparative examples was 12.1, which is high as compared to that ofeach example.

That is, since a value of the maximum texture intensity is small, andthe contour is widely spread in examples, it can be derived that thecrystal grains with various orientations are distributed.

On the other hand, since a value of the maximum texture intensity islarge, and the contour is concentrated in comparative examples, it canbe appreciated that a large amount of crystal grains in the<0001>/C-axis direction is included.

From the above results, it can be appreciated that the magnesium alloysheets of examples have a more excellent formability.

This may be appreciated through FIGS. 8 and 9 of the present invention.

FIG. 8 shows a result of electron backscatter diffraction (EBSD)analysis of the magnesium alloy sheet produced in Example 1a.

FIG. 9 is a graph illustrating fractions of crystal orientations ofExample 1a.

First, as illustrated in FIG. 8, the crystal orientation of crystalgrain may also be measured by EBSD. More specifically, the crystalorientation of crystal grain may be measured by EBSD by injectingelectrons into a specimen through e-electron beam and using inelasticscattering diffraction at the back of the specimen.

In addition, as illustrated in FIG. 9, crystal grains having amisorientation angle of 20° or less between grains may be bottom crystalgrains. Therefore, it was confirmed that the volume fraction of thecrystal grains in the <0001>/C-axis direction relative to 100% of thevolume fraction of the entire crystal grains was about 18.5%.

In addition, as illustrated in FIG. 8, it can be appreciated that thecrystal grains with various orientations were distributed in variouscolors, and the crystal grains (red) corresponding to the crystal grainsin the <0001>//C-axis direction could be confirmed with naked eyes fromthe EBSD results.

TABLE 2 Size of crystal Sheet Yield Limiting dome grain thicknessstrength height (μm) (mm) (MPa) (LDH, mm) Example 1a 19 0.7 164 9.4Example 1b 7 0.6 161 8.2 Example 1c 6 1 166 8.1 Example 1d 13 1 155 7.5Example 1e 21 1 157 8 Example 1f 25 1 154 9.9 Example 1g 16 0.7 151 9Example 1h 15 3 155 9.1 Example 1i 17 1 164 9 Comparative 10 0.7 188 2.5Example 1a Comparative 11 0.6 155 5 Example 1b Comparative 40 1.5 1455.1 Example 1c Comparative 8 1 166 4.9 Example 1d

As a result, it was confirmed that in Comparative Examples of 1a to 1dwhich did not satisfy the conditions of homogenization annealing time,rolling temperature, and intermediate annealing temperature, theformability was inferior to that of each example. In addition, it can beappreciated that a yield strength of each of Comparative Examples of 1ato 1d was inferior to that of each example. In Comparative Example 1c,an average size of the crystal grains was about 40 μm, that is, theformability was relatively excellent as compared to that of the othercomparative examples, but a level of the formability was less than thatof each example.

Example 2

A molten metal containing, relative to 100 wt % of the entire moltenmetal, 3.0 wt % of Al, 0.1 wt % of Zn, 1.0 wt % of Ca, 0.3 wt % of Mn,and the balance of Mg and inevitable impurities was prepared.

The molten metal was casted to prepare a magnesium cast material.

The magnesium cast material was subjected to a primary homogenizationheat treatment at 350° C. for 10 hours. The magnesium cast materialsubjected to the primary homogenization heat treatment was subjected toa secondary homogenization heat treatment at 450° C. for 10 hours.

A rolled material was prepared by casting the cast material subjected tohomogenization heat treatment.

Thereafter, the rolled material was subjected to subsequent heattreatment at 400° C. for 10 hours.

Finally, a magnesium sheet was produced by subjecting the rolledmaterial subjected to the subsequent heat treatment to skin pass, andthe skin pass temperature and reduction ratio are as shown in Table 2.

Comparative Example 2

A magnesium alloy sheet was produced in the same manner as that ofExample 2, except for the conditions of skin pass temperature andreduction ratio.

In order to compare and evaluate physical properties of the magnesiumalloy sheets produced in examples and comparative examples, thefollowing experimental examples were performed In addition, anexperimental example for measurement of the limiting dome height andanalysis of the crystal orientation was carried out, and theexperimental method is the same as described above.

Experimental Example 4: Comparison of Physical Properties Depending onReduction Ratio of Skin Pass and Temperature

TABLE 3 Maximum Skin pass Skin pass Yield tensile Limiting temperatureReduction strength strength Elongation dome height (C.) ratio (%) (MPa)

 (MPa) rate (%) (LDH, mm) Example 2a 250 5 202 257 22 8.1 Example 2b 9211 254 22 8.0 Example 2c 15 252 272 16 7.3 Comparative 22 272 289 8.67.0 Example 2a Comparative 300 X 140 233 23 8-9 Example 2b DELETEDTEXTSExample 2d 7 203 253 22 8   Example 2e 12 247 267 18 7.3 Comparative 17247 272 10 7.3 Example 2c

As shown in Table 3, as a result of subjecting the magnesium alloyshaving the same compositions and components as each other, a yieldstrength was improved without a large change of the formability. Morespecifically, the formability may be measured by comparing numericalvalues of an elongation rate and a limiting dome height.

In addition, the formability may be secured by minimizing the change ofthe texture, and the change of the texture depending on the reductionratio of the skin pass may be confirmed through FIG. 10.

FIG. 10 is a result of EBSD analysis of a magnesium alloy sheetdepending on a reduction ratio of skin pass.

As illustrated in FIG. 10, it can be confirmed that even in a case wherethe skin pass after rolling was further performed, the crystal grainswith various orientations were distributed. In addition, in a case wherethe rolling was performed in a state in which the reduction ratio of theskin pass was increased, the orientation change of the texture wasminimized, and the strength of the magnesium alloy sheet was improved,due to the development of the twinned crystal (black) structure andpotential.

Specifically, it was confirmed that in a case where the reduction ratioof the skin pass is 2 to 6%, the area fraction of the twinned crystalstructure, relative to 100% of the entire area of the magnesium alloy,was 15%. It was confirmed that in a case where the reduction ratio ofthe skin pass is 6 to 15%, the area fraction of the twinned crystalstructure, relative to 100% of the entire area of the magnesium alloy,was 30%.

As described above, due to the twinned crystal structure and potential,the strength of the magnesium alloy sheet may be maintained and theformability of the magnesium alloy sheet may also be improved.

Therefore, in a case where the rolling is performed by exceeding thereduction ratio of 15% (Comparative Example 2a), the texture of the(0001) surface is developed again, which causes deterioration of theformability of the magnesium alloy sheet.

FIG. 11 shows a maximum texture intensity of each of (0001) surfaces ofExample 2 and Comparative Example 2, depending on a skin pass condition.

As illustrated in FIG. 11, even in a case where the skin pass wasperformed, the change of the texture of examples was not large. However,it could be appreciated that in a case where the reduction ratio of theskin pass was excessive as in Comparative Example 2a, the intensity ofthe texture was largely changed. Therefore, as illustrated in Table 3,it was confirmed that a phenomenon in which an increase effect of theyield strength was excellent, but the elongation rate significantlydeteriorated.

In addition, it was also confirmed that an increase effect of the yieldstrength depending on the reduction ratio of the skin pass wassignificantly exhibited as compared to an increase effect of the yieldstrength depending on the change in skin pass temperature.

Although the embodiments of the present invention has been describedwith reference to the accompanying drawings, those skilled in the artwill appreciate that various modifications and alterations may be madewithout departing from the spirit or essential feature of the presentinvention.

Therefore, it should be understood that the aforementioned embodimentsare illustrative in terms of all aspects and are not limited. The scopeof the present invention is defined by the appended claims rather thanthe detailed description, and all changes or modifications derived fromthe meaning and scope of the appended claims and their equivalentsshould be interpreted as falling within the scope of the presentinvention.

1. A magnesium alloy sheet comprising: relative to 100 wt % of theentire magnesium alloy sheet, 2.7 to 5.0 wt % of Al, 0.75 to 1.0 wt % ofZn, 0.1 to 1.0 wt % of Ca, 1.0 wt % or less of Mn (excluding 0 wt %),and the balance of Mg and other inevitable impurities, wherein a volumefraction of bottom crystal grains is 30% or less relative to 100 vol %of overall crystal grains of the magnesium alloy sheet, and wherein thebottom crystal grains are crystal grains in a <0001>//C-axis direction.2. The magnesium alloy sheet of claim 1, wherein: the magnesium alloysheet comprises Al—Ca secondary phase particles, and a difference inarea fraction of the Al—Ca secondary phase particles is 10% or lessbetween a quarter portion (¼) of a surface of the magnesium alloy sheetand a center portion (½) of the surface of the magnesium alloy sheet. 3.The magnesium alloy sheet of claim 2, wherein: a ratio of a length ofcenter segregation to a total length of the magnesium alloy sheet in arolling direction is less than 5%.
 4. The magnesium alloy sheet of claim3, wherein: a ratio of a thickness of the center segregation to a totalthickness of the magnesium alloy sheet in a thickness direction is lessthan 2.5%.
 5. The magnesium alloy sheet of claim 4, wherein: a limitingdome height (LDH) of the magnesium alloy sheet is 7 mm or more, and amaximum texture intensity of a (0001) surface of the magnesium alloysheet is 1 to
 4. 6. A magnesium alloy sheet comprising: relative to 100wt % of the entire magnesium alloy sheet, 2.7 to 5.0 wt % of Al, 0.75 to1.0 wt % of Zn, 0.1 to 1.0 wt % of Ca, 1.0 wt % or less of Mn (excluding0 wt %), and the balance of Mg and other inevitable impurities, whereina volume fraction of a twinned crystal structure is 35% or less relativeto 100 vol % of the entire area of the magnesium alloy sheet.
 7. Themagnesium alloy sheet of claim 6, wherein: the volume fraction of thetwinned crystal structure, relative to 100 vol % of the entire area ofthe magnesium alloy sheet, is 5 to 35%.
 8. The magnesium alloy sheet ofclaim 7, wherein: a volume fraction of bottom crystal grains is 30% orless relative to 100 vol % of overall crystal grains of the magnesiumalloy sheet, and the bottom crystal grains are crystal grains in a<0001>//C-axis direction.
 9. The magnesium alloy sheet of claim 8,wherein: a limiting dome height of the magnesium alloy sheet is 7 mm ormore, and a maximum texture intensity of a (0001) surface of themagnesium alloy sheet is 1 to
 4. 10. The magnesium alloy sheet of claim9, wherein: a yield strength of the magnesium alloy sheet is 200 to 300MPa.